Modern wireless mobile communication systems present two notable features: one is broad band and high rate—for example, the bandwidth of the fourth generation wireless mobile communication system may reach 100 MHz, and its downlink rate may be up to 1 Gbps; and the other is mobile interconnection, which has promoted emerging services like mobile Internet-browsing, mobile video-on-demand, and on-line navigation, etc. These two features call for advanced wireless mobile communication technologies, such as ultra high rate wireless transmission, inter-region interference suppression, reliable signal transmission in mobile environments, distributed/centralized signal processing, etc. In the enhanced 4th generation (4G) and the 5th generation (5G) wireless mobile communication systems of the future, various corresponding key technologies have been proposed and discussed to meet the above development requirements, which deserves extensive attention from researchers in the art.
In October of 2007, the International Telecom Union (ITU) has approved the Worldwide Interoperability for Microwave Access (WiMAX) as the fourth 3G system standard. This event, which happened at the end of the 3G era, is in fact a rehearsal of the 4G standard war. Indeed, in order to confront the challenges from the wireless IP technology represented by wireless local area network (WLAN) and WiMAX, the 3GPP organization has set out to prepare for its new system upgrade—standardization of the Long Term Evolution (LTE) system. As a quasi-4G system which is based on Orthogonal Frequency Division Multiplexing (OFDM), the LTE system had its first release published in 2009, and was subsequently put into commercial use in 2010. Meanwhile, the standardization of the 4G wireless mobile communication system was also started by 3GPP in the first half of 2008, and this system is referred to as Long Term Evolution Advanced (LTE-A). The critical standard specification for physical layer procedures of that system was completed in early 2011. In November of 2011, the ITU officially announced in Chongqing, China that the LTE-A system and the WiMAX system are two official standards for 4G systems. Nowadays, global commercialization of the LTE-A system is progressing step by step.
Although the 4G wireless mobile communication systems, represented by the LTE-A system and the WiMAX system, are able to provide users with communication services at higher rates and enhance users' experience with the services, they are still not capable of sufficiently meeting user demands in the next few years or decade. Currently, mobile communication systems serve approximately 5.5 billion users, and it is estimated that this number will rise up to 7.3 billion in 2015. This involves a significant increase in the number of smartphone users—in 2011 there were about 0.428 billion smartphones in the world, while in 2015 this number will be doubled to about 1 billion. The popularization of powerful smartphones has promoted a rapid increase in wireless mobile communication rate. In recent years, the wireless communication rate steadily doubles every year in the worldwide range. At this increasing rate, in 10 years from now, the rate of wireless mobile communication systems will have to be increased by 1000 times as compared with that of current systems to accommodate users' basic requirements for communication rates. In general, the rate mentioned above mainly refers to that of data services, which account for approximately 90% of the total traffic and include for example downloading of smartphone applications, real-time navigation, cloud based synchronization and sharing of personal information, etc. The traffic of voice services, in comparison, is not likely to increase dramatically in the next decade due to relatively slow population growth.
In addition to the challenge of increasing the wireless communication rate by 1000 times, another challenge arises from the burgeoning of mobile Internet. Currently, 70 percent of Internet accesses are initiated from mobile terminals. The next decade would be a new opportunity period for the IT industry and the major opportunity lies in that the conventional PC Internet would be gradually replaced by the mobile Internet. Then, new user habits would hasten the emergence of new service modes, such as software developing for handheld communication devices and touch screens, individual-location based social network, individual oriented cloud based information management, etc. The mobile Internet impacts the wireless mobile communication systems mainly in two aspects. First, mobile video data traffic will increase significantly, and it is expected to occupy about 66% of the overall data traffic by 2016. Due to their relatively high level of real-time property, such services as mobile video raise a higher reliability requirement for the wireless mobile communication systems. Second, in the future, most mobile data communications will occur indoors or in hotspot cells, which will also bring challenge to the coverage of the wireless mobile communication system.
Moreover, in 2020, there will be 20 billion machine-to-machine communication devices in the world, and their data traffic will increase to 500% of the current level. How to design systems to support numerous machine-to-machine communication devices is also a topic that needs to be explored in depth.
According to the challenges of the next decade, requirements for the development of the enhanced 4G wireless mobile communication system are generally as follows:                Pursuing for higher wireless broadband rates, with focus placed on optimization of local hotspot cells;        Further improving user experience, with communication services on cell edges particularly optimized;        Continuing researches on new technologies that can improve spectrum utilization efficiency, considering that it is impossible for the available spectrum to be expanded by 1000 times;        Putting into use higher frequency bands (5 GHz or higher) to obtain broader communication bandwidth;        Coordinating existing networks (2G/3G/4G, WLAN, WiMAX, etc.) to share the burden of data traffic;        Optimizations specific to different services and applications;        Strengthening systems' abilities to support massive machine-to-machine communications;        Flexible, intelligent and low cost network planning and deploying;        Devising schemes to save power consumption of networks and battery consumption of user equipments.        
To meet the above requirements, in June of this year, a special working conference was held by 3GPP in Slovenia to discuss key technologies of the enhanced 4G wireless mobile communication system. In this conference, a total of 42 proposals were published and discussed, and there were three major key technologies finally adopted: Enhanced Small Cell, 3D (three dimensional) MIMO (multiple in multiple out) and Enhanced Coordinated Multi-Point communication.
Among them, the coordinated multi-point communication technology is a technology by which multiple users receive communication services from one or more TPs. Specifically, the technology enables the system to collect a user's channel state information (CSI) from distributed TPs and perform multi-point coordination and resource allocation, so as to satisfy the user's QoS requirements and make efficient use of various resources of the network as a whole. It is to be noted that the so-called TP refers to a set of multiple transmission ports corresponding to a group of downlink CSI-RS patterns, and is not limited to a conventional base station.
During the past decade, researches on coordinated multi-point communications have been flourishing, due to quick maturation of the theory of MIMO system. In MIMO systems, generalized models for uplink and downlink channels are typically referred to as a multiple access channel (MAC) model and a broadcast channel (BC) model. As is proved theoretically, there is a duality between the MAC model and the BC model in capacity domain when a UE is equipped with a single antenna, and this holds true even when multiple antennas are mounted at both the transmitter and the receiver. As is further proved theoretically, the duality still exists when each of the antennas at the transmitter and the receiver is subject to an independent power constraint. That is to say, if the antennas are grouped and the groups of antennas are virtually considered as transmitting nodes or receiving nodes, the MAC model or the BC model of a MIMO system may evolve into a coordinated multi-point communication network and may reach the upper limit of its capacity. After the fundamental problem on the upper limit of capacity was solved, researches have been carried out extensively on the coordinated multi-point communication technology, which and thus becomes a hotspot of academic research.
In industrial standardization, the coordinated multi-point communication technology receives extensive attentions as well and has been preliminarily applied to the 4G wireless mobile communication system. For example, in the LTE-A systems, a scheme called Coordinated Multi Points (CoMP) has been adopted and is mainly applied in downlink. Additionally, multi-TP joint reception in uplink is an optional scheme for TPs. However, in the 4G wireless mobile communication systems, only the non-coherent CoMP scheme, which is easy to implement, is considered.
With regard to the multi-antenna multi-TP coordination, an outline is available from the standard document: 3GPP TR 36.819 V2.0.0 (2011 September), “Coordinated Multi-Point Operation for LTE physical layer aspects (Release 11)”, which can be summarized as follows:                To receive a multi-antenna multi-TP service, a UE needs to report, for a set of TPs, channel state/statistical information of a link between the UE and each of the set of TPs. This set of TPs is referred to as a measurement set for multi-antenna multi-TP transmission.        The set of TPs for which the UE actually performs information feedback can be a subset of the measurement set and is referred to as a coordination set for multi-antenna multi-TP transmission. Obviously, the coordination set for multi-antenna multi-TP transmission can be the same as the measurement set for multi-antenna multi-TP transmission.        A TP in the coordination set for multi-antenna multi-TP transmission can participate in transmission of Physical Downlink Shared Channel (PDSCH, which is the UE's data channel) to the UE, either directly or indirectly.        The scheme in which multiple TPs directly participate in coordinated transmission is referred to as Joint Processing (JP). The JP scheme requires PDSCH signal intended for the UE to be shared among the multiple TPs participating in the coordination and can be subdivided into two approaches. One is referred to as Joint Transmission (JT) in which the multiple TPs transmit their PDSCH signals to the UE simultaneously. The other one is referred to as Dynamic Point Selection (DPS) and Dynamic Point Blanking (DPS), in which at any time only one of the TPs which has the strongest signal link is selected to transmit its PDSCH signal to the UE while signal links of the other TPs are disabled.        The scheme in which multiple TPs indirectly participate in coordinated transmission is referred to as Coordinated Beamforming/Coordinated Scheduling (CB/CS). In the CB/CS scheme, beams/resources for transmitting PDSCHs to different UEs are coordinated among the multiple TPs to suppress interference between each other. It is not required to share PDSCH signal intended for the UE among the multiple TPs participating in the coordination.        For a UE operating in the coordinated multi-antenna multi-TP transmission environment, information feedback is mainly carried out separately for each TP and is transmitted over uplink resources of the serving TP.        
As used herein, the term “MIMO” refers to a wireless transmission technology in which multiple antennas are deployed at both the transmitter and the receiver to make use of spatial resources in wireless transmission and thus achieve spatial multiplex gain and spatial diversity gain. Researches on information theory have shown that the capacity of a MIMO system grows linearly with the minimum of the number of transmitting antennas and the number of receiving antennas. FIG. 1 shows a schematic diagram of a MIMO system. As shown in FIG. 1, a plurality of antennas at the transmitter and the receiver constitute a multi-antenna wireless channel containing spatial domain information.
In addition, the term “information feedback” mainly refers to a process in which a UE feeds back CSI to a TP such that the TP can perform corresponding operations such as radio resource management. There are primarily the following three CSI feedback approaches in the prior art.
Complete CSI Feedback:
The UE quantizes all elements in a transceiver channel matrix and feeds back each of the elements to the TP. Alternatively, the UE can analogously modulate all elements in the transceiver channel matrix and feeds them back to the TP. Alternatively, the UE can obtain a transient covariance matrix for the transceiver channel matrix, quantizes all elements in the covariance matrix and feeds back each of the elements to the TP. Thus, the TP can reconstruct an accurate channel from the channel quantization information fed back from the UE. This approach is described in detail in non-patent document 1: 3GPP R1-093720, “CoMP email summary”, Qualcomm and its implementation is illustrated in FIG. 2.
Statistic-Based CSI Feedback:
The UE applies a statistical process on a transceiver channel matrix, e.g., calculating a covariance matrix thereof, quantizes the statistical information and then feeds it back to the TP. Thus, the TP can obtain statistical state information of the channel based on the feedback from the UE. This approach is described in detail in non-patent document 1: 3GPP R1-093720, “CoMP email summary”, Qualcomm and its implementation is illustrated in FIG. 3.
CSI Feedback Based on Codebook Space Search:
A finite set of CSI (i.e., a codebook space) is predefined by the UE and the TP. Generally, a typical codebook space includes channel rank and/or precoding matrix and/or channel quality indications, etc. Upon detection of a transceiver channel matrix, the UE searches the codebook space for an element best matching the CSI of the current channel matrix and feeds back the index of the element to the TP. Thus, the TP looks up the predefined codebook space based on the index to obtain rough CSI. This approach is described in detail in non-patent document 2: 3GPP, R1-083546, “Per-cell precoding methods for downlink joint processing CoMP”, ETRI, and its implementation is illustrated in FIG. 4.
Among the above three approaches, the complete CSI feedback has the best performance, but is difficult to be applied in actual systems due to the highest feedback overhead. In particular, in the coordinated multi-antenna multi-TP system, its feedback overhead grows in proportional to the increase of the number of TPs and it is even more difficult to implement. The CSI feedback based on codebook space search has the lowest feedback overhead, but is worst in terms of performance since it cannot accurately describe the channel state such that the transmitter cannot make full use of channel characteristics to perform transmission accordingly. However, it is extremely simple to implement and can typically accomplish feedback with a few bits. Hence, it is widely applied in actual systems. In comparison, the statistic-based CSI feedback achieves a good trade-off between these two approaches. When the channel state information exhibits remarkable statistical properties, this approach can accurately describe the channel state with a relatively small amount of feedback, thereby achieving a relatively ideal performance.
Currently, in LTE and LTE-A systems, in consideration of factors influencing practical system implementation, the CSI feedback based on codebook space search is employed in the single-cell transmission mode. In the coordinated multi-TP mode of the LTE-A system, it is expected that this CSI feedback based on codebook space search will continue to be used. Therefore, in the following, the present invention will be described with respect to this feedback approach.
For the CSI feedback based on codebook space search, there are two feedback channels in the LTE system, namely, a Physical Uplink Control CHannel (PUCCH) and a Physical Uplink Shared CHannel (PUSCH). In general, the PUCCH is configured for transmission of periodic, basic CSI with low payload; while the PUSCH is configured for transmission of bursty, extended CSI with high payload. On the PUCCH, a complete CSI transmission involves various feedback contents which are transmitted in different sub-frames. On the PUSCH, on the other hand, a complete CSI transmission is carried out within one sub-frame. Such design principles remain applicable in the LTE-A system.
The feedback contents can be divided into three categories: Channel Quality Index (CQI), Precoding Matrix Index (PMI) and Rank Index (RI), all of which are bit quantized feedbacks. The CQI typically corresponds to a transmission format having a packet error rate not more than 0.1.
In the LTE system, the following eight types of MIMO transmission schemes for downlink data are defined:                1) Single antenna transmission, which is used for signal transmission at a single-antenna TP. This scheme is a special instance of a MIMO system and can only transmit a single layer of data.        2) Transmission diversity. In a MIMO system, the effects of time and/or frequency diversities can be utilized for transmitting signals, so as to improve the reception quality of the signals. This scheme can only transmit a single layer of data.        3) Open-loop space division multiplexing, which is a space division multiplexing that does not require PMI feedback from UE.        4) Closed-loop space division multiplexing, which is a space division multiplexing that requires PMI feedback from UE.        5) Multi-user MIMO. Multiple UEs participate in downlink communications of a MIMO system on the same frequency at the same time.        6) Closed-loop single layer precoding, which is enabled by using a MIMO system. Only a single layer of data is transmitted, and PMI feedback from UE is required.        7) Beam forming transmission, which is enabled by using a MIMO system and the beam forming technique. A dedicated reference signal is configured for data demodulation at a UE. Only a single layer of data is transmitted, and PMI feedback from UE is not required.        8) Two-layer beam forming transmission. A UE may or may not be configured to or not to feed back PMI and RI.        
In order to support the above MIMO transmission schemes, a variety of CSI feedback modes are defined in the LTE system. Each MIMO transmission scheme corresponds to a number of CSI feedback modes, as detailed in the following.
On the PUCCH, there are four CSI feedback modes applicable, namely, Mode 1-0, Mode 1-1, Mode 2-0 and Mode 2-1. These modes are combinations of four feedback classes, including:                1) Class 1, which relates to a preferred sub-band (SB) location in a Band Part (BP, which is a subset of a set of communication spectrum resources (denoted as Set S) and has a size dependent on the size of the Set S) and CQI(s) for the SB. The respective overheads are L bits for the SB location, 4 bits for the CQI of a first codeword and 3 bits for the CQI of a possible second codeword. The CQI of the second codeword is differentially coded with respect to the CQI of the first codeword.        2) Class 2, which relates to broadband CQI(s) and a PMI. The respective overheads are 4 bits for the CQI of a first codeword, 3 bits for the CQI of a possible second codeword and 1, 2 or 4 bits for the PMI depending on the antenna configuration at the TP. The CQI of the second codeword is differentially coded with respect to the CQI of the first codeword.        3) Class 3, which relates to an RI. The overhead for the RI is 1 bit for two antennas or 2 bits for four antennas, depending on the antenna configuration at the TP.        4) Class 4, which relates to a broadband CQI. The overhead is constantly 4 bits.        
In accordance with the above different classes, the UE correspondingly feeds back different information to the TP.
Mode 1-0 is a combination of Class 3 and Class 4. That is, feedbacks in accordance with Class 3 and Class 4 are carried out at respective periods and/or with respective sub-frame offsets. This means the broadband CQI of the first codeword on the Set S and possibly the RI information are fed back.
Mode 1-1 is a combination of Class 3 and Class 2. That is, feedbacks in accordance with Class 3 and Class 2 are carried out at respective periods and/or with respective sub-frame offsets. This means the broadband PMI on the Set S, the broadband CQIs of the individual codewords and possibly the RI information are fed back.
Mode 2-0 is a combination of Class 3, Class 4 and Class 1. That is, feedbacks in accordance with Class 3, Class 4 and Class 1 are carried out at respective periods and/or with respective sub-frame offsets. This means the broadband CQI of the first codeword on the Set S and possibly the RI information, the preferred SB location in the BP and the CQI on the SB are fed back.
Mode 2-1 is a combination of Class 3, Class 2 and Class 1. That is, feedbacks in accordance with Class 3, Class 2 and Class 1 are carried out at respective periods and/or with respective sub-frame offsets. This means the broadband PMI on the Set S, the broadband CQIs of the individual codewords and possibly the RI information, the preferred SB location in the BP and the CQI on the SB are fed back.
Between the MIMO transmission schemes and the CSI feedback modes, there are correspondence relationships as follows:                MIMO transmission scheme 1): Mode 1-0 and Mode 2-0;        MIMO transmission scheme 2): Mode 1-0 and Mode 2-0;        MIMO transmission scheme 3): Mode 1-0 and Mode 2-0;        MIMO transmission scheme 4): Mode 1-1 and Mode 2-1;        MIMO transmission scheme 5): Mode 1-1 and Mode 2-1;        MIMO transmission scheme 6): Mode 1-1 and Mode 2-1;        MIMO transmission scheme 7): Mode 1-0 and Mode 2-0;        MIMO transmission scheme 8): Mode 1-1 and Mode 2-1, in case the UE feeds back PMI/RI; or                    Mode 1-0 and Mode 2-0, in case the UE does not feed back PMI/RI.                        
In the single TP transmission mode of the LTE-A system, CQI, PMI and RI are also the primary feedback contents. Moreover, in order to keep the feedback modes for a UE consistent with those corresponding to the transmission schemes 4) and 8) and to enable new transmission schemes 9) and 10), which are dynamic MIMO switching (wherein the TP can dynamically adjust the MIMO mode in which the UE operates) and CoMP transmission (wherein multiple TPs communicate in a coordinated manner) respectively, in the LTE-A system emphasis is placed on optimizing Mode 1-1 and Mode 2-1 for a scenario where a TP is equipped with 8 transmission antennas. Specifically, a PMI is determined jointly by two channel precoding matrix indices W1 and W2, with W1 indicating broadband/long-term channel characteristics and W2 indicating SB/short-term channel characteristics. For transmitting W1 and W2 over PUCCH, Mode 1-1 is sub-divided into two sub-modes: Mode 1-1 (sub-mode 1) and Mode 1-1 (sub-mode 2). The original Mode 2-1 is also modified.
In order to support the newly defined feedback modes, several feedback classes are newly defined in the LTE-A system as follows:                1) Class 1a, which relates to a preferred SB location in a Band Part (BP, which is a subset of a set of communication spectrum resources (denoted as Set S) and has a size dependent on the size of the Set S) and a CQI on the SB as well as a W2 on another SB. The overhead for the SB location is L bits, and the total overhead for the CQI and the W2 is 8 bits (if RI=1), 9 bits (if 1<RI<5), or 7 bits (if RI>4).        2) Class 2a, which relates to a W1. The overhead for the W1 is 4 bits (if RI<3), 2 bits (if 2<RI<8), or 0 bits (if RI=8).        3) Class 2b, which relates to a broadband W2 and a broadband CQI. The total overhead for the broadband W2 and the broadband CQI is 8 bits (if RI=1), 11 bits (if 1<RI<4), 10 bits (if RI=4), or 7 bits (if RI>4).        4) Class 2c, which relates to a broadband CQI, a W1 and a broadband W2. The total overhead for the broadband CQI, the W1 and the broadband W2 is 8 bits (if RI=1), 11 bits (if 1<RI<4), 9 bits (if RI=4), or 7 bits (if RI>4). It is to be noted that, in order to limit the feedback overhead, the set of values from which the W1 and the broadband W2 can take their values is formed by down-sampling a complete set of all possible values of the W1 and the broadband W2 (namely, the former set of values is a subset of the latter set of values).        5) Class 5, which relates to an RI and a W1. The total overhead for the RI and the WI is 4 bits (for 8 antennas and 2-layer data multiplexing) or 5 bits (for 8 antennas and 4/8-layer data multiplexing). Also, it is to be noted that, in order to limit the feedback overhead, the set of values from which the W1 can take its value is formed by down-sampling a complete set of all possible values of the W1.        6) Class 6, which relates to an RI and a Precoding Type Indicator (PTI). The overhead for the PTI is 1 bit, indicating the type of precoding. The total overhead for the RI and the PTI is 2 bits (for 8 antennas and 2-layer data multiplexing), 3 bits (for 8 antennas and 3-layer data multiplexing), or 4 bits (for 8 antennas and 8-layer data multiplexing).        
Here, “W1” and “W2” when used alone refer to “broadband W1” and “broadband W2” respectively, while “SB W2” is referred to by its full name.
The mode-class relationships between Mode 1-1 (sub-mode 1), Mode 1-1 (sub-mode 2) and Mode 2-1 and the original and the above new feedback classes are as follows:                Mode 1-1 (sub-mode 1) is a combination of Class 5 and Class 2b. That is, feedbacks in accordance with Class 5 and Class 2b are carried out at respective periods and/or with respective sub-frame offsets.        Mode 1-1 (sub-mode 2) is a combination of Class 3 and Class 2/2c,                    when the MIMO transmission scheme is of type 4) or 8), Mode 1-1 (sub-mode 2) is composed of Class 3 and Class 2. That is, feedbacks in accordance with Class 3 and Class 2 are carried out at respective periods and/or with respective sub-frame offsets.            when the MIMO transmission scheme is of type 9) or 10), Mode 1-1 (sub-mode 2) is composed of Class 3 and Class 2c. That is, feedbacks in accordance with Class 3 and Class 2c are carried out at respective periods and/or with respective sub-frame offsets.                        the new Mode 2-1 is specific to the MIMO transmission scheme of type 9) or 10), and is a combination of Class 6, Class 2b and Class 2a/1a,                    when the PTI related to Class 6 is 0, the new Mode 2-1 is composed of Class 6, Class 2b and Class 2a. That is, feedbacks in accordance with Class 6, Class 2b and Class 2a are carried out at respective periods and/or with respective sub-frame offsets.            when the PTI related to Class 6 is 1, the new Mode 2-1 is composed of Class 6, Class 2b and Class 1a. That is, feedbacks in accordance with Class 6, Class 2b and Class 1a are carried out at respective periods and/or with respective sub-frame offsets.                        
Further, it is to be noted that the TSG-RAN WG1 meeting #71 was held in New Orleans, United States, in November 2012. According to the minutes of the meeting, a CSI process is defined to be determined by a CSI reference signal resource (CSI-RS-R) and an interference measurement resource (IMR). That is, the signal portion of the CSI process is determined by a measurement for the CSI-RS-R and the interference portion of the CSI process is determined by a measurement for the IMR. In the CoMP transmission, a TP configures multiple CSI processes for a UE, and the RI of a CSI process may be configured to be the same as the RI of another CSI process. Specifically, a CSI process is defined as an RI reference process, and the TP may configure another RI dependent CSI process to inherit the RI of the RI reference process and have it reported so as to facilitate the CoMP transmission. In particular, in the new Mode 2-1, both the RI and the PTI of the RI reference process are inherited by the RI dependent process. In the new Mode 1-1 (sub-mode 1), because the RI and the W1 are reported at the same time, both the RI and W1 of the RI reference process are inherited by the RI dependent process if the RI reference process and the RI dependent process have their RI and W1 reported at the same time, and only the RI of the RI reference process is inherited by the RI dependent process if the RI reference process and the RI dependent process have their RI and W1 reported at different times. It is to be noted that, in order to ensure that the inheritance is practicable, the feedback mode and the number of antenna ports for the RI dependent process must be the same as those for the RI reference process.
The RI inheritance is mainly used to effect non-coherent JT and frequency domain DPS/DPB operations, because in the LTE and LTE-A systems the RI for a data transmission of a UE shall remain constant in frequency domain (see 3GPP RI-124625, “Rank and Subband Inheritance between CSI Processes”, Ericsson).
In the enhanced 4G system of the future, it is necessary to make researches on transmission schemes such as non-coherent JT to develop for example new CSI feedback designs, TP selection algorithms, precoding and power allocation algorithms and the like. Among these, relative CSI between TPs (such as information of a relative phase between TPs) or aggregated CQI for TPs are important feedback contents for supporting non-coherent JT transmission (see 3GPP R1-121349, “Comparison between inter-CSI-RS co-phase and aggregated CQI”, SHARP). Besides, in order to support non-coherent JT better, the frequency domain SB inheritance needs to be considered, so that signals from multiple TPs can be superposed coherently at the same frequency.
Therefore, in the enhanced 4G system of the future, how to design a feedback method compatible with existing mechanisms to support various CoMP transmissions is an important research topic.